my sc400 is doing the same thing it runs rough on idle. ive either got a small blown head gasket which sometimes have no symptoms but exist anyway or a valve isnt opening all the way because i can hear a lifter tapping. test your coil packs and try changing the igniter. it controls the dewll of your spark to (on time) with tdc and if its bad it might make one plug fire at the slightest wrong time. and fuel cleaner is a bad idea. if one plug is burning lean and the others arent thats an injector issue.

 

You put a new MAF sensor and it's still got a CEL on for bad MAF sensor. Obviously, your problem is a wire integrity problem. Check for an open/short in the MAF sensor circuit. Trace from the MAF sensor all the way to the PCM for broken wires. It's gonna be quite a task.